Hitherto, it has been known that the pyridazin-3-one compounds given in the formula (I): ##STR3## wherein, X represents a hydrogen or halogen atom; Y represents a halogen atom; R.sup.2 represents a hydrogen atom or C.sub.1 -C.sub.3 alkyl group; R.sup.3 represents a C.sub.1 -C.sub.3 haloalkyl group; B represents a hydrogen atom, OR.sup.4 group or COOR.sup.5 group; R.sup.4 represents a hydrogen atom, C.sub.1 -C.sub.6 alkyl, C.sub.3 -C.sub.6 alkenyl, C.sub.3 -C.sub.6 alkynyl, (C.sub.1 -C.sub.8 alkoxy)carbonylC.sub.1 -C6 alkyl or carboxyC.sub.1 -C.sub.6 alkyl group; and R.sup.5 represents a hydrogen atom, C.sub.1 -C.sub.6 alkyl, C.sub.3 -C.sub.6 alkenyl or C.sub.3 -C.sub.6 alkynyl group,
and their analogs have excellent herbicidal activity (WO 97/07104), and it has been demanded to develop a simple and convenient method to produce these pyridazin-3-one compounds. PA1 in the case of producing the herbicidal pyridazin-3-one compounds given in the above formula (I). Further, the present invention serves a method of producing the pyridazin-3-one compounds given in the above formula (I). The method comprises i) reacting the hydrazone compound of the formula (III): ##STR5## wherein, X, Y, B, R.sup.1 and R.sup.3 represent the same definitions as above, with the phosphorane compound given in the formula (IV): EQU (C.sub.6 H.sub.5).sub.3 P=C(R.sup.2)COOR.sup.6